Conditional discrimination learning by pigeons traditionally has been thought to involve acquisition of a series of stimulus-response chains of sample-specific rules of the form: respond to B after having been stimulated by A, and respond to D after having been stimulated by C, where A and C are conditional stimuli that indicate whether responses to B or D will be reinforced Recent evidence suggests that (1) conditional discrimination learning may involve not only A-B, and C-D approach learning but also, A-D, C-B avoidance learning and (2) similarity (identity) between the sample stimuli (A and C) and the comparison stimuli (B and D) can be an important aspect of the task. The proposed research will examine what is learned when pigeons acquire a conditional discrimination and will determine the role played by stimulus, response, and reinforcer relations. The goal of the research is to develop a more complete theory of conditional discrimination learning.